Historically, voice communications systems have been based on a plesiochronous digital hierarchy (PDH) in which transmitting devices and receiving devices nominally operate at the same rate, but the transmitting and receiving devices may not be perfectly synchronized. In North America, the PDH includes 64 kbps (DS0), 1.544 Mbps (DS1), 6.312 Mbps (DS2) and 44.736 Mbps (DS3) signals. Signals used in Europe and Japan are slightly different.
Service providers are increasingly using synchronous optical network (SONET) equipment to transmit data using fiber optic technologies. Various components of the SONET and PDH standards are described in Telcordia Technologies, “Synchronous Optical Network (SONET) Transport System: Common Generic Requirements, GR-253-core,” Issue 3, September 2000, Telcordia Technologies, “Transport System Generic Requirements (TSGR): Common Requirements GR-499-core,” Issue 2, December 1998, and American National Standards Institute (ANSI) T1.105.03, “Synchronous Optical Network (SONET): Jitter at Network Interfaces,” 1994.
Increasing use of SONET equipment results in integration of PDH and SONET networks. This integration can result in situations in which PDH traffic is carried by SONET equipment where processing (e.g., multiplexing) of signal is performed on PDH signals. Various techniques have been developed to perform these operations. However, some current techniques are inefficient.